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My PC has two connections (wireless and wired) simultaneously. Both of them have an internet connection.

In this situation, how does it connect to the internet? How do my LAN and WLAN interfaces know to send requests to the web?

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Are you asking which connection is being used? Which operating system are you using? –  The_aLiEn Feb 15 '12 at 3:19
    
Connected both of them. Yes! –  KevinOelen Feb 15 '12 at 3:21
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4 Answers

up vote 8 down vote accepted

If multiple interfaces are up there are two issues. First one is which interface has gateway and what is the interface order.

I'm assuming you're using Windows O.S. In command prompt (type "cmd" in Run window) type route print and you should see something like this:

routeprint

(the headers are in you o.s. language, mine is Turkish)

Which interface has the gateway option, actually which interface has the routing information for destination 0.0.0.0, mask 0.0.0.0 is the actual outgoing door...

{ Edit: Observe that I have a route line that is

Destination: 192.168.8.0 Mask: 255.255.248.0 and Gateway 192.168.24.2

at the bottom. That says when i need to access 192.168.8.0 network, i have to pass throug the 24.2 gateway. Besides, there are no entries in main table for 192.168.24.0 network, that was an experimental entry over another NIC }

When you have multiple route informations for that destination {edit: 0.0.0.0} and each one is different from each other or just two different gateways, you're doomed. Network will start losing some packets...

When you have multiple route informations for that destination but one gateway for each (which is your case that your gateway will be your modem i guess, which is your dhcp server, it'll assing different ip addresses for each interface but same information for default dns and default gateway) then there is an order:

order

Your o.s. will treat your up and running interfaces with this ordering. So if you wireless interface is on the first line and connected, your o.s. will use that interface for network communication and vice-versa.

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That second screenshot is of the Adapters and Bindings window, which can be brought up in Windows 7. Go to the Network Connections window, press ALT once to bring up the menu bar, go to Advanced > Advanced Settings.... –  Bob Feb 15 '12 at 4:53
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I was wondering this myself when I installed a wireless NIC in my system. I did some test (in Windows XP) and found out that if you have both a wired and wireless connection (both independently configured and functioning), then Windows will prioritize the wired connection.

In other words, when the cable is disconnected, it will use the wireless connection, but when you plug in the network cable, the wireless connection is essentially stopped (not disabled, just no longer used, that is, zero bytes transfered in either direction) and the wired connection is used instead.

When my mother got a laptop, I repeated the test with it (in Windows 7) and got the same results.

For the record, this behavior makes sense. A wired connection will be faster and more secure than a wireless one. Why use the wireless one if the wired one works?

How does[sic] my Lan and Wlan interfaces know to send request to web?

The network adapters do not decide that, the operating system does. Windows decides which interface to send and receive from.

(I could not test it, but I do not believe that the choice of which adapter to use depends on the speed. I suppose it could use a faster wireless connection over a slower wired one, but that would be a bizarre setup indeed. Also, note that I am talking about a wired vs. wireless connection as per the question, I am not discussing multiple wired connections and redundancy. In the case that the prioritized wried connection goes down, obviously Windows switches over to the wireless one.)

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There's probably some other method since i use a gig-e wired connection between two systems (non internet) and wireless internet. Its not speed related. –  Journeyman Geek Feb 15 '12 at 5:42
    
No, I wouldn’t imagine so. I suspect that it first prioritizes wired connections over wireless ones, and then in those two sub-groups, it uses the order that The_aLiEn described. –  Synetech Feb 15 '12 at 5:46
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Sorry if you already found the answer, but this is possible and is really nice.

Cisco routers are able to make two connections to one device, depending on the types of connections it's either unequal cost path load balancing, or equal cost path load balancing. The router will transfer the data through both connections, it'll increase file transfers, maybe downloads depending on your connection to the modem, what your plan with the ISP is, but it should increase your download speed, most slow connections are a result of the wireless connection. You usually find this being used with servers, or NAS devices, usually where multiple users or other devices are sending/receiving data from the storage devices. If it's set up properly, it will make a noticeable difference. If you want maximum speed for a laptop, you're best bet is getting a Cisco Gigabit router with wireless-N, or wireless-AC, unless you're using a desktop, then two cat6 connections would be... just.... wow. Hope this helps

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It is unlikely that both interfaces are simultaneously active. When one interface is active, the other(s) are inactive. Windows will send traffic through the default active connection. Your machine will, unless you've done some deliberate configuration, use only one interface at a time.

That said, you can have two interfaces active in a computer, but they will have to be on different subnets to function successfully. The internet-facing interface will send non-local packets out to the public network; the LAN-facing interface will send packets locally on the local area network. This is how routers work: they examine packets and determine based on IP address which interface to send packets out of.

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